The Canon EF-S 55-250mm f/4-5.6 IS STM Telephoto Zoom Lens is a long-reaching zoom lens that provides a 35mm-equivalent focal length range of 88-400mm, covering portrait-length to telephoto perspectives to suit working with distant subject matter. Greatly benefitting this lens’ reach is the incorporation of image stabilization, which works to minimize the appearance of camera shake by up to 3.5 shutter speed steps to support handheld use in low-light conditions.

An STM stepping focus motor is also integrated into the lens’ design to provide smooth, quiet, and fast focusing performance that is ideally-suited to video applications as well as tracking moving subjects. The rear focusing system, high-speed CPU, and an enhanced AF algorithm also contribute to quickened overall AF speeds. Full-time manual focus is also supported to permit fine-tuning of focus even while working in AF modes. Additionally, the front lens section does not rotate during focusing to better facilitate the use of polarizing filters.

The optical construction of the lens employs one ultra-low dispersion glass element to help reduce chromatic aberrations and distortion throughout the zoom range while also enhancing contrast and clarity. Optimized lens coatings have also been applied to ensure color fidelity by countering the effects of lens flare and ghosting.

It's a promising piece of glass, now seriously starting to threaten the 70-300 IS for crop owners.

I'd say the previous (non-STM) versions of the 55-250 were already a better choice for crop users than the 70-300 non-L. This lens is significantly better, with FTM and a non-rotating front element.

Having had both the 55-250 and 70-300 non L the only difference I have noticed is the 70-300 seems to focus a little faster, a slightly better build and the greater focal length. IQ seems to be pretty close. I do agree with your assessment that the STM model is a a better choice.

How does STM compare with USM? (Not in technical details, but in how it performs)

As a practical matter, STM makes for faster and quieter autofocusing. But that is only one aspect of this new lens as compared to its predecessor. The internal/rear focusing, nonrotating front element, upgraded CPU and electronics, and full manual focus are nice improvements IMHO. The closest focus distance is nearly a foot less than the previous version (2.8 ft vs 3.6 ft). Plus the filter size is the same as the previous version (58mm), so I don't need to buy new filters. And despite all these improvements, the new version weighs 15 grams less than its predecessor (375g vs. 390g). The images it produces will tell the whole story, but on the surface, it looks like this lens was improved in every way that really matters.